Just to let people know about a project I've been working on: MikeOS. It's a small 16-bit real mode operating system for x86 PCs, written entirely in assembler. It's not planned to become a big fully-fledged OS -- more a learning tool for people interested in x86 assembly language and basic OS design.
Current features include: FAT12 floppy disk reading; menu-based program selector; command-line interface; 26 system calls (eg screen and string handling); rudimentary DOS compatibility; an API for developers; example programs and more. The DOS compatibility is improving to the stage that I can run simple COM games and the first release of VisiCalc
Downloads and screenshots here:
http://mikeos.berlios.de
Cheers,
Mike